1. Field of the Invention
The present invention relates to a surface-mount type quartz crystal oscillator, and in particular, to a surface-mount type crystal oscillator that allows an oscillation frequency to be adjusted during manufacture by irradiating a quartz crystal blank with ion beams.
2. Description of the Related Arts
A surface-mount type quartz crystal oscillator includes a quartz crystal blank and an IC (Integrated Circuit) chip including an oscillation circuit using the crystal blank, and the crystal blank and IC chip are accommodated in a container. Owing to the small size and light weight thereof, the surface-mount type crystal oscillator is incorporated specifically into portable electronic equipment as a reference source for frequency and time. To be set to a desired value for the crystal oscillator, the oscillation frequency can desirably be adjusted during manufacture. Thus, in some crystal oscillators, the crystal blank is irradiated with ion beams during manufacture to vary a resonance frequency of the crystal blank so as to allow the oscillation frequency to be adjusted.
FIG. 1A is a sectional view of a conventional surface-mount type crystal oscillator, and FIG. 1B is a plan view of the crystal oscillator with a cover removed therefrom.
The surface-mount type crystal oscillator uses container body 3 with a recess in which IC chip 1 and crystal blank 2 are accommodated. Metal cover 4 is placed over container body 3 to close the recess to hermetically encapsulate IC chip 1 and crystal blank 2 in container body 3. IC chip 1 has integrated electronic circuits making up an oscillation circuit using crystal blank 2. IC chip 1 includes at least an amplification element for oscillation. A configuration of the oscillation circuit will be described below. It is assumed that a Colpitts oscillation circuit is constructed here.
In IC chip 1, the electronic circuits are formed on one principal surface of a semiconductor substrate by a normal semiconductor device fabrication process. Thus, the one of the principal surfaces of the semiconductor substrate on which the electronic circuits are formed is hereinafter referred to as a “circuit formation surface” of the IC chip. A plurality of IC terminals 6 are also formed on the circuit formation surface to connect IC chip 1 to an external circuit. IC terminals 6 are arranged in two rows on the generally rectangular circuit formation surface along paired long sides thereof. IC terminals 6 include pairs of connection terminals 6x, 6y for use in electric connection to crystal blank 2. IC terminals 6 further include a power supply terminal, an output terminal, a ground terminal, and a standby terminal. Connection terminals 6x, 6y are provided on opposite sides of one end of the circuit formation surface of IC chip 1 in a longitudinal direction thereof.
As shown in FIG. 2, crystal blank 2 is a generally rectangular, AT-cut quartz crystal blank including excitation electrodes 7x, 7y on opposite principal surfaces thereof. Lead-out electrodes 8x, 8y extend from paired excitation electrodes 7x, 7y, respectively, toward an outer peripheral portion of crystal blank 2. Here, lead-out electrodes 8x, 8y extend toward opposite sides of one end of crystal blank 2, that is, opposite ends of one short side. Each of lead-out electrodes 8x, 8y is formed to be folded back between the principal surfaces of crystal blank 2 at the position of the end of crystal blank 2.
In illustrated crystal blank 2, when lead-out electrodes 8x, 8y extend downward in the figures from excitation electrodes 7x, 7y on the respective principal surfaces, lead-out electrode 8x connected to excitation electrode 7x on the illustrated front-side principal surface extends to a right end of a lower side of crystal blank 2. In crystal blank 2, the excitation electrodes and the lead-out electrodes are formed to be rotationally symmetric about a longitudinal central line of crystal blank 2. Thus, even if crystal blank 2 is turned upside down, the lead-out electrode still extends to the right end of the lower side. As described below, lead-out electrodes 8x, 8y are electrically connected to paired connection terminals 6x, 6y on IC chip 1.
FIG. 3 is a circuit diagram showing a circuit configuration of this crystal oscillator. In this figure, crystal blank 2 is depicted by a circuit symbol of a crystal element. IC chip 1 includes an inverter element as amplification element 5 for oscillation made up of, for example, a C-MOS (complementary MOS (metal-oxide-semiconductor)) element. Moreover, resistor R and split capacitors Ca, Cb are formed on IC chip 1. Resistor R is provided between an input end and an output end of amplification element 5 as feedback resistance. Crystal blank 2 is also connected between the input end and out end of amplification element 5. Here, excitation electrode 7x and lead-out electrode 8x are connected to the output end of amplification element 5. Excitation electrode 7y and the lead-out electrode 8y are connected to the input end of amplification element 5. Capacitor Ca is provided between the output end of amplification element 5 and a ground point. Capacitor Cb is provided between the input end of amplification element 5 and the ground point. Oscillation output Vout is obtained from the output end of amplification element 5. In FIG. 3, the elements located outside a dotted frame are provided inside IC chip 1.
Container body 3 is a flat, generally rectangular parallelepiped and is made up of laminated ceramic. A generally rectangular recess is formed in one principal surface of container body 3 to accommodate IC chip 1 and crystal blank 2. Two step portions 9a, 9b are formed on an inner wall of the recess. The height of the recess from an inner bottom surface thereof is such that first step portions 9a are lower than second step portions 9b. First step portions 9a are formed along opposite long sides of the recess. Second step portions 9b are formed along opposite short sides of the recess. A plurality of circuit terminals 10 for connection to IC terminals 6 of IC chip 1 are provided on a top surface of each of first step portions 9a. Paired holding terminals 11x, 11y for electric connection to crystal blank 2 are provided on a top surface of second step portion 9b provided at one end of the recess. Here, second step portion 9b provided at the one end of the recess may be divided into two pieces so that holding terminals 11x, 11y are independent of each other.
IC chip 1 is secured to the inner bottom surface of container body 3 so that the circuit formation surface faces upward. Each of IC terminals 6 on the circuit formation surface is electrically connected, by wire bonding using lead wire 12 such as a gold (Au) wire, to a corresponding one of circuit terminals 10, provided on first step portions 9a. In this case, IC chip 1 is located on the inner bottom surface of the recess so that the end thereof with connection terminals 6x, 6y formed thereat is located closer to the step portion with holding terminals 11x, 11y formed thereon. Those of the circuit terminals provided on both first step portions 9a which are closest to holding terminals 11x, 11y, that is, circuit terminals 10x, 10y, are connected to connection terminals 6x, 6y, respectively, of IC chip 1 by wire bonding. Holding terminals 11x, 11y, provided on second step portion 9b, are electrically connected to circuit terminals 10x, 10y via through-holes 15 or the like.
Mounting terminals 14 for use for surface-mounting the crystal oscillator on a wiring board are provided in four corners of an outer bottom surface of the container body. Circuit terminals 10 connected to the power supply, output, ground, and standby terminals, included in IC terminals 6, are electrically connected to mounting terminals 14 via through-holes and conductive paths formed through lamination plane between ceramic layers in container body 3.
Crystal blank 2 is held in the recess of container body 3 and electrically connected to IC chip 1, by securing paired lead-out electrodes 8x and 8y to holding terminals 10x, 10y, respectively, with conductive adhesive 13 at positions where paired lead-out electrodes 8x, 8y are led out. In this case, in the recess, crystal blank 2 is positioned above IC chip 1 so as to cover IC chip 1.
Such a crystal oscillator is completed by securing IC chip 1 to the inner bottom surface of the recess, carrying out the wire bonding, then securing crystal blank 2 to holding terminals 10x, 10y, and thereafter joining metal cover 4 to the top surface of container body 3 at a position where metal cover 4 surrounds the opening of the recess, to close the recess so that IC chip 1 and crystal blank 2 are hermetically encapsulated in container body 3. In the crystal oscillator, one of the excitation electrodes of crystal blank 2, that is, excitation electrode 7x, located opposite metal cover 4, is electrically connected to the output end of amplification element 5 in IC chip 1.
For the above-described crystal oscillator, the oscillation frequency is adjusted as follows. Crystal blank 2 is secured to holding terminals 10x, 10y. Then, power is actually supplied to the crystal oscillator to perform an oscillation operation. Excitation electrode 7x is irradiated with ion beams from an ion gun (not shown) with the oscillation frequency monitored. A sputtering effect of the ion beams reduces the mass of excitation electrode 7x to increase the oscillation frequency. Thus, excitation electrode 7x is irradiated with the ion beams until the oscillation frequency reaches a predetermined value, to adjust the oscillation frequency. Once the adjustment of the oscillation frequency is completed, metal cover 4 may be joined to container body 3 to complete the crystal oscillator. The principal surface of crystal blank 2 which is irradiated with the ion beams is not located opposite IC chip 1, that is, is located opposite metal cover 4 when the recess is closed by metal cover 4.
In the above-described example, excitation electrode 7x, irradiated with the ion beams, is connected to the output end of amplification element 5. Thus, charges generated by the irradiation with the ion beams flow into the ground point via amplification element 5 to avoid affecting the oscillation operation. Here, as described in Japanese Patent Laid-Open No. 2001-244744 (JP-2001-244744A), when the excitation electrode irradiated with the ion beams is connected to the input end of amplification element 5, charges generated by the ion beam irradiation are accumulated at the input end to change an operating point of amplification element 5. As a result, a phenomenon such as stoppage of the oscillation may occur. When the oscillation is thus stopped, the adjustment of the oscillation frequency cannot be continued.
The arrangement of the IC terminals on IC chip 1 may vary with the type of the IC chip based on a design of the IC chip vendor. Even when the condition that the paired connection terminals for connection to the crystal blank are provided at the end of IC chip 1 is met, which of the output and input ends of the amplification element connects to each of the two connection terminals may vary depending on the vendor's design. For example, in the above description, connection terminal 6x located in a lower left corner of the configuration shown in FIG. 1B is connected to the output end of the amplification element. However, connection terminal 6x may be connected to the input end depending on the design of the IC chip.
Now, in the configuration shown in FIG. 1B, it is assumed that connection terminal 6x located in the lower left corner of IC chip 1 in the figure is connected to the output end of amplification element 5. Then, if the crystal blank shown in FIG. 2 is used, then during frequency adjustment, excitation electrode 7x, electrically connected to the output end of amplification element 5, is irradiated with ion beams. This avoids affecting the oscillation operation. However, if connection terminal 6x located in the lower left corner of IC chip 1 is connected to the input end of amplification element 5, the excitation electrode electrically connected to the input end of amplification element 5 is irradiated with ion beams. Thus, the oscillation operation is affected.
To deal with the fact that the paired connection terminals on the IC chip may be connected to the amplification element for oscillation in two manners, that is, the connection terminal may be connected to either the input side or output side of the amplification element, two crystal blanks 2 shown in FIGS. 4A and 4B in which the lead-out electrodes extend from the excitation electrodes in different directions to each other crystal blank may be provided so that one of crystal blanks 2 can be used depending on the type of IC chip 1. Here, the crystal blank shown in FIG. 4A is identical to the one shown in FIG. 2. The crystal blank shown in FIG. 4B is a symmetrical mirror image of the one shown in FIG. 4A. That is, in FIG. 4B, lead-out electrode 8x extends downward from excitation electrode 7x in the figure and thus extends toward the left end of the lower side of crystal blank 2.
In the configuration shown in FIG. 1, if an IC chip is used in which connection terminal 6x located in the lower left corner of the IC chip in the figure is connected to the input end of the amplification element, the crystal blank shown in FIG. 4B is used and not the one shown in FIG. 4A. Then, the excitation electrode irradiated with ion beams is electrically connected to another connection terminal 6y, which is connected to the output end of the amplification element. Thus, the oscillation operation can be prevented from being affected.
As described above, the oscillation frequency can be adjusted regardless of whatever IC chip is used, by using one of the two types of crystal blanks depending on the arrangement of the paired connection terminals on the IC chip. However, in this case, two types of crystal blanks in a mirror image relationship need to be in stock. Furthermore, which of the crystal blanks is to be used needs to be selected during manufacture. This may deteriorate productivity.